1. Field of the Invention
The present invention relates to a direct current machine, such as a direct current motor.
2. Description of Related Art
One previously known direct current machine, such as a direct current motor, includes magnets of different polarities (N-pole and S-pole), an armature, a commutator and two brushes. The direct current motor is rotated when a direction of electric current, which is supplied to each corresponding armature coil, is switched through the brushes and the commutator.
At the time of switching the direction of electric current, i.e., at the time of commutation, due to inductance of the armature coils, a negative effect, which tends to retard a linear change of the electric current, occurs to cause insufficient commutation. In such a case, the electric current, which flows from the commutator to the armature coils, could be forcefully stopped at a late stage of the commutation to cause generation of sparks (commutation sparks). This phenomenon is known to cause brush wearing, noise generation, electromagnetic noise generation or the like. Thus, it has been demanded to address such a disadvantage.
Japanese Unexamined Patent Publication No. 2002-95230 addresses the above disadvantage and discloses a direct current motor, which achieves improved commutation by changing distribution of the magnetic flux (i.e., distribution of magnetic flux density) of each magnet to change the amount of magnetic flux, which passes across each corresponding armature coil that is under the commutation process.
FIG. 21 shows a direct current motor 71 recited in Japanese Unexamined Patent Publication No. 2002-95230. Specifically, a motor housing 77 receives a pair of magnets 72, 73. The magnets 72, 73 form an N-pole and an S-pole, respectively, and are opposed to one another about an armature 74. The armature 74 includes an armature core 78, armature coils 79a, 79b and a commutator 80. Twelve teeth 78a are formed in the armature core 78. Each armature coil 79a, 79b is wound around a corresponding group of six teeth 78a. Although not depicted, each of the rest of armature coils is similarly wound around a corresponding group of six teeth 78a. 
The commutator 80 is arranged at one end of the armature 74. The commutator 80 includes a plurality of segments (commutator segments) 76. Adjacent two segments 76a, 76b are connected to one another by the corresponding armature coil 79a, and adjacent two segments 76c, 76d are connected to one another by the corresponding armature coil 79b. Brushes 75a, 75b are urged against the commutator 80 to slidably engage the commutator 80. Direct current, which is supplied from a direct current power source (not shown), is applied to the armature coils 79a, 79b through the brushes 75a, 75b and the corresponding segments 76 of the commutator 80.
When the armature 74 is rotated in a direction of X, the segments 76a, 76b are short circuited by the brush 75a, so that short circuit electric current flows through the armature coil 79a. Furthermore, the segments 76c, 76d are also short circuited by the brush 75b, so that short circuit electric current flows through the armature coil 79b. During the short circuiting by the corresponding brush 75a, 75b, direction of electric current, which flows in the corresponding armature coil 79a, 79b, is switched, so that the armature 74 is further rotated in the clockwise direction (in the direction of X in FIG. 21). As shown in FIG. 21, the twelve segments 76 are arranged at 30 degree intervals in the circumferential direction, and the direction of the electric current in each corresponding armature coil 79a, 79b is switched. More specifically, the commutation process of the armature coil 79a, 79b is performed during 30 degree rotation of the armature 74.
The magnet 72 includes a main magnetic pole 72a (N-pole) and two end magnetic poles 72b, 72c (S-poles). The end magnetic poles 72b, 72c extend from opposite ends, respectively, of the main magnetic pole 72a. The magnet 73 includes a main magnetic pole 73a (S-pole) and two end magnetic poles 73b, 73c (N-poles). The end magnetic poles 73b, 73c extend from opposite ends, respectively, of the main magnetic pole 73a. During a first half of the commutation process, a leading end 78b of the leading end tooth, which is wound with the armature coil 79a under the commutation process, is located at the end magnetic pole 72b of the magnet 72, which is located in a front end of the magnet 72 in the rotational direction of the armature 74. During a last half of the commutation process, the leading end 78b is located at the end magnetic pole 73c of the magnet 73, which is located in a rear end of the magnet 73 in the rotational direction of the armature 74. Furthermore, during the first half of the commutation process, a leading end 78b of the leading end tooth, which is wound with the armature coil 79b under the commutation process, is located at the end magnetic pole 73b of the magnet 73, which is located in a front end of the magnet 73 in the rotational direction of the armature 74. During the last half of the commutation process, the leading end 78b is located at the end magnetic pole 72c of the magnet 72, which is located in a rear end of the magnet 72 in the rotational direction of the armature 74.
With the above arrangement, in the first half of the commutation process, the magnetic flux, which passes through the armature coil 79a, is reduced by the end magnetic pole 72b (the S-pole), which has the polarity opposite from that of the main magnetic pole 72a (the N-pole). In the last half of the commutation process, the magnetic flux, which passes through the armature coil 79a, is increased by the end magnetic pole 73c (the N-pole), which has the same polarity as that of the main magnetic pole 72a (the N-pole). Furthermore, in the first half of the commutation process, the magnetic flux, which passes through the armature coil 79b, is reduced by the end magnetic pole 73b (the N-pole), which has the polarity opposite from that of the main magnetic pole 73a (the S-pole). In the last half of the commutation process, the magnetic flux, which passes through the armature coil 79b, is increased by the end magnetic pole 72c (the S-pole), which has the same polarity as that of the main magnetic pole 73a (the S-pole). Thus, in the first half of the commutation process, induced voltage, which is generated due to a change in the magnetic flux that passes through the armature coil 79a, 79b under the commutation process, acts in a commutation retarding direction for retarding the commutation. Contrary to this, in the last half of the commutation process, induced voltage, which is generated due to a change in the magnetic flux that passes through the armature coil 79a, 79b under the commutation process, acts in a commutation facilitating direction for facilitating the commutation. In this way, the commutation can be improved.
Furthermore, it has been also proposed to form the magnets 72, 73 in such a manner that the end magnetic pole 72b and the end magnetic pole 73c are directly connected to one another, and the end magnetic pole 72c and the end magnetic pole 73b are directly connected to one another in the above direct current motor 71. In this way, the magnets 72, 73 extend along the entire inner peripheral surface of the motor housing 77, so that the magnetic flux of the magnets 72, 73 can be effectively used.
In the above direct current motor 71, the magnetic flux is changed by providing the end magnetic poles 72b, 72c, 73b, 73c in the opposite ends, respectively, of each main magnetic pole 72a, 73a to improve the commutation. However, in the direct current motor 71, the end magnetic pole 72b (the S-pole), which has the polarity opposite from that of the main magnetic pole 72a (the N-pole), is provided in the front circumferential end portion of the magnet 72 located in the front end of the magnet 72 in the rotational direction of the armature 74. Also, the end magnetic pole 73b (the N-pole), which has the polarity opposite from that of the main magnetic pole 73a (the S-pole), is provided in the front circumferential end portion of the magnet 73 located in the front end of the magnet 73 in the rotational direction of the armature 74. In order to reduce the amount of magnetic flux, which passes through the armature coil 79a, 79b under the commutation process, the amount of magnetic flux, which contributes to rotation of the armature 74 is disadvantageously reduced.
Furthermore, in general, in the direct current motor, when electric current flows through the armature coil, a magnetic flux is generated by armature magnetomotive force, so that the magnetic flux of the permanent magnets is influenced. This phenomenon is known as armature reaction. When the armature reaction is relatively large, spatial distribution of magnetic flux is substantially distorted. For example, when the induced voltage is increased by increasing the electric power supplied to the armature coil, the switching of the commutation electric current is delayed. Thus, at the end of the commutation process, electric current can be abruptly switched to cause generation of brush sparks. In order to limit this phenomenon, it has been demanded to reduce the influence of the armature reaction even in a case where electric power supplied to each corresponding armature coil is increased.
However, in the above direct current motor 71, the influence of the armature reaction at the time of increasing the electric power supplied to each corresponding armature coil 79a, 79b is not concerned. Thus, it has been demanded to limit inhibition of effective commutation by the armature reaction even in the case where the electric power supplied to the armature coil 79a, 79b is increased.